The invention concerns a procedure to determine the oxygen storage capability of an emission control system with an exhaust gas sensor with a discrete-level ability disposed in front of the emission control system. The emission control system can, for example, concern a three-way catalytic converter, which is deployed in a vehicle with a gasoline engine in order to transform toxic exhaust gas components into harmless gases through an oxidation or reduction reaction. The storage capability for oxygen of an emission control system is utilized for the purpose of taking in oxygen in the lean phases and giving it off again in the rich phases. As a result of this even the toxic gas components of the exhaust gas, which are to be oxidized, can be converted. The lambda value one will thus be constantly measured at the outlet of the emission control system. Hence, a stoichiometric balance of the oxidizing and reducing agents occurs. With an increasing deterioration of the emission control system, its storage capability for oxygen reduces. As a result oxygen can no longer be sufficiently provided in the rich phases to purge the exhaust gas of toxic gas components, and the lambda sensor behind the emission control system detects these components to be oxidized. Furthermore, this lambda sensor detects the oxygen in longer lean phases, which can no longer be stored by the emission control system. In many countries a test of the emission control system during vehicle operation is legally mandatory using an engine management system (On-board-Diagnosis). A known diagnostic procedure is to determine the oxygen storage capability of the emission control system, as according to experience the conversion capability also decreases with the storage capability.
A known procedure is based upon initially displacing the lambda value in front of the emission control system in the rich direction to a lambda value smaller than 1 and during this phase to remove all of the oxygen out of the catalytic converter. In a subsequent lean phase with a lambda value greater than 1, the catalytic converter is loaded with the excess oxygen until exhaust gas containing oxygen appears at its outlet. The amount of oxygen put in storage up to that point is the storage capability of the catalytic converter.
In the German patent DE 41 12 478 C2 a procedure to appraise the degree of deterioration of a catalytic converter is described, in which
the lambda values in front of and behind the catalytic converter are measured;
it is examined, if in the case of a norm oscillation in front of the catalytic converter from rich to lean or vice versa, the lambda value behind the catalytic converter shows a corresponding transition and then, if this is the case;
the gas quantity stream flowing through the catalytic converter is determined;
the chronological integral of the product from the gas quantity stream and the lambda value behind the catalytic converter are calculated;
and as a unit of measurement for the degree of deterioration either
the difference between the two integrals
or the quotient from the two integrals
or the quotient from the difference and from one of the two integrals is used.
A disadvantage of the procedure described is that the lambda value in front of the emission control system has to be measured with a complex wideband lambda sensor in order to decide on the amount of oxygen introduced or extracted by way of the integration of the product from the actual lambda value and gas quantity stream.
An additional procedure to determine the oxygen storage capability of an emission control system is described in the patent EP 0546 318 B1. The system is acted upon by a lambda progression, whose oxygen deficiency input at the beginning is higher than the oxygen storage capability of the catalytic converter. The oxygen input is so selected, that the catalytic converter in the lean phases is in each instance filled to its capacity limit. To determine the oxygen storage capability of the catalytic converter, the median lambda value in front of the catalytic converter during the lambda oscillations of the system is displaced intentionally in the lean direction and in so doing the oxygen extraction is reduced from phase to phase. By determining the number of rich-lean-transitions, which the lambda sensor disposed behind the catalytic converter indicates, the oxygen storage capability can be determined, whereby an increased number of phases means a reduced storage capability.
A disadvantage to this procedure is that toxic exhaust gas is released in the lean phases.
The German patent DE 3816558 A1 discloses a procedure to control the lambda value of the air/fuel-mixture to be supplied to an internal combustion engine with the help of a medium for the two-step control with specified closed-loop control parameters, to which (the medium) the signal of the control lambda sensor, which has a discrete-level ability, is to be supplied for the formation of a control deviation is thereby characterized, in that                for the respective operating state at hand a lambda measurement set point is established,        the median lambda value is used as the lambda measurement actual value,        the measurement deviation between the lambda measurement set point and the lambda measurement actual value is calculated and        at least one control parameter as a function of the measurement deviation is altered in such a way, that a lambda measurement actual value should arise, which reduces the stated measurement deviation.        
The procedure indeed applies to a lambda sensor with a discrete-level ability in front of an emission control system and mentions an averaging of the lambda value, does not, however, apply to an open-loop control of the lambda value for the testing of an emission control system.